Corneal transplantation is the oldest and most common form of solid organ transplantation. In the U.S., over 40,000 corneal transplants are performed each year. Although the success rate of corneal transplants is high, 10-15 percent will fail due to immune rejection. In these patients, the risk for graft failure soars to a 65-75 percent for subsequent corneal grafts. Immunosuppressive drugs often fail in such "high-risk" hosts and even when they prevent rejection, they carry serious side effects including renal and hepatic toxicity. Thus, new and less toxic immunosuppressive strategies are needed for use in "high-risk" hosts or in hosts who cannot tolerate current immunosuppressive drugs. This project will address two specific aims: (1) identify and characterize the immune mechanisms responsible for corneal allograft rejection, and (2) develop and evaluate two novel strategies for preventing the immune rejection of corneal allografts. Both aims will be addressed in a well-characterized mouse model of penetrating keratoplasty. Experiments will employ perforin knockout mice to determine the role of cytotoxic T cells in corneal graft rejection. Other experiments will utilize selective reconstitution of perforin knockout mice to determine the role of delayed-type hypersensitivity and tumor necrosis factor-alpha (TNF-alpha) in mediating corneal graft rejection. The second specific aim will develop and evaluate the two promising novel strategies for preventing the immune rejection of corneal allografts: (1) oral immunization with donor-specific cells, and (2) blockade of costimulatory pathways of T cell activation. Since oral immunization produces a profound inhibition of corneal graft rejection, studies will identify the optimal protocol, longevity of graft enhancement and the mode of action of antigen feeding. Other investigations will determine the efficacy of blocking costimulatory pathways of T cell activation and its effect on corneal allograft survival. Both strategies procure antigen-specific suppression of host immune responses and should prove useful in preventing corneal graft rejection.